Report of examination of the samples from core shroud (2F3-H6a) at Fukushima Dai-ni Nuclear Power Station Unit-3 (Contract research)

At the Fukushima Dai-ni Nuclear Power Station Unit-3 of The Tokyo Electric Company (Boiling Water Reactor, 1100 MW), cracks were observed near welded joint portion of outside surface of lower ring of core shroud by the visual testing of in-core structures in the 11th periodical inspection from April to July, 2001. Core shroud has a cylindrical geometry located in reactor pressure vessel and its function is to support the core structure and to make coolant flow route from lower to upper portion inside the core shroud by jet pumps in normal operation. The present examination has been performed with the objective to ensure the transparency of the examination as the third-party organization by providing technical basis for identifying the causes of cracking through examination of the sample taken from the cracked region of outer H6a welding portion of the core shroud at Fukushima Dai-ni Nuclear Power Station Unit-3, which was a part of sample stored in the Nippon Nuclear Fuel Development Co., Ltd. in the examination of Tokyo Electric Power Company in 2001. The present examination of the sample was conducted at the post irradiation examination facilities of JAERI (the Research Hot Laboratory (RHL), etc.). The following findings were obtained from the result of the present examination. (1) Three cracks were observed at the portion 3 to 9 mm1 apart from the weld metal and the maximum depth was about 8 mm. (2) From the result of observation of two fracture surfaces, intergranular cracking was observed in almost whole fracture surface. The transgranular cracking was partially observed within the depth of about 300 μm from the surface. (3) Hardening layer over Hv400 at its maximum was found from the surface to the depth of about 500 μm. Softening by welding process was observed in the surface layer of about 3 mm from the weld metal. (4) The oxides were observed in the whole inside of cracks from opening portion to the crack tip. The oxides mainly consisted of Fe-oxides. (5) Slight fluctuations of contents of main elements in the alloys were observed, but no significant correlation was confirmed between the oxides and the cause of cracking. Based on the examination results described above concerning presence of tensile residual stress by welding and relatively high dissolved oxygen contents in core coolant and so on, it is concluded that the cracks were mainly initiated in the hardening layer of outer H6a welding portion of core shroud by transgranular stress corrosion cracking (SCC) and propagated along the grain boundaries. (author)